A large bioburden survey of outdated units of refrigerator stored red blood cells (rbc) was performed by the Upjohn Company during 1989-1991, in which a total of 11,702 rbc pools, representing 585,100 units, were tested for bioburden using a standard pour plate culture method designed to detect a general spectrum of bacterial and fungal contaminants. Of the 70 positive cultures isolated, none were identified as Y. enterocolitica or any related species. To determine the sensitivity of the method employed for the specific detection of Y. enterocolitica, a validation study was carried out using stored samples of rbc pools obtained directly from Upjohn. Twenty samples of rbc pools were inoculated with 105 104 and 103 organisms per ml from a culture of Y. enterocolitica grown in whole blood. The inoculated pools were cultured by the identical method employed in the Upjohn survey. All inoculated pools yielded the predicted colony counts based upon the known input inoculum. It was concluded that the presence of Y. enterocolitica in any of the rbc pools surveyed in the Upjohn study would have been detected. These data confirmed the rarity of rbc contamination by Y. enterocolitica, and contributed to a decision by the Blood Products Advisory Committee not to recommend reduced dating for refrigerator stored RBC's.